1) Field of the Invention
The present invention relates to a driving apparatus of a light-emitting device.
2) Description of the Related Art
Recently, an optical signal is used for signal transmission via a communication line and network, because the optical signal offers advantages such as enlarging the capacity of information carried and accelerating the information transmission speed. An electrical signal output from various information equipment such as a computer and a telephone is converted to the optical signal in an optical communication apparatus and supplied to an optical communication line and network. A laser diode (LD), which has extremely high frequency response and is easily miniaturized, is widely used for a photoelectric conversion device included in the optical communication apparatus.
In order to carry out a stabile optical communication, it is necessary to constantly monitor an optical output from the laser diode and to control an operation of the laser diode. In general, the laser diode for the optical communication is accompanied by an optical feedback circuit so as to automatically adjust a driving current. The optical feedback circuit includes components such as a photodiode for monitoring backward light (optical output) of the laser diode, a current-to-voltage conversion circuit for converting an output current from the photodiode to a voltage, an operational amplifier section, and a reference voltage circuit section. The aforementioned circuit is referred to as an APC (Automatic Power Control) circuit, since the circuit automatically stabilizes the optical output from the laser diode.
The optical communication apparatuses including the APC circuits are disclosed in Japanese Patent Kokai No. 6-232917, Japanese Patent Kokai No. 2000-174827 and Japanese Patent Kokai No. 2001-36470.
When an offset voltage of the operational amplifier included in a driving control circuit for the laser diode increases, the control of the driving current flowing through the laser diode becomes unstable, because an optical amplitude value or an optical peak value can not be accurately detected. The offset voltage is caused by, for example, piece-to-piece variation of the operational amplifiers or a fluctuation of an ambient temperature around the optical communication apparatus. When a driving bias current far exceeds a light-emitting threshold value, a light-emitting state remains during an off period of the light output from the laser diode, and thus an optical noise increases during the off period. Accordingly, an on/off ratio (extinction ratio) is deteriorated. On the other hand, when the driving bias current decreases, a transient response time from an optical turning off to an optical turning on is delayed, thereby causing a delay in the light output from the laser diode.
In this connection, when a capacitor is connected in series to an output terminal of the operational amplifier included in the APC circuit, a DC (direct current) component is cut off from a current output from the operational amplifier, thereby extracting only an amplitude component. Consequently, the offset voltage of the operational amplifier does not affect the control of the driving current for the laser diode.
However, in a conventional APC circuit with the series-connected capacitor, when a probability of appearances of “1” and “0” in each transmitted frame data signal significantly fluctuates, a DC potential after the series-connected capacitor varies, and thus the control of the driving current flowing through the laser diode becomes unstable. Therefore, the laser diode including the aforementioned APC circuit can not be used for the transmissions other than the transmission of a balanced-sign, in which the probability of appearances of “1” and “0” in each transmitted frame data signal is constant. Accordingly, the laser diode including the aforementioned APC circuit is not suitable for a stable output of a signal having a changing probability of appearances of “1” and “0” such as a burst signal.